Engineers typically design high-pressure oil field plunger pumps in two sections; the (proximal) power section and the (distal) fluid section. The power section usually comprises a crankshaft, reduction gears, bearings, connecting rods, crossheads, crosshead extension rods, etc. The power section is commonly referred to as the power end by the users and hereafter in this application. The fluid section is commonly referred to as the fluid end by the users and hereafter in this application. Commonly used fluid sections usually comprise a plunger pump housing having a suction valve in a suction bore, a discharge valve in a discharge bore, an access bore, and a plunger in a plunger bore, plus high-pressure seals, retainers, etc. FIG. 1 is a cross-sectional schematic view of a typical fluid end showing its connection to a power end by stay rods. FIG. 1 also illustrates a fluid chamber which is one internal section of the housing containing the valves, seats, plungers, plunger packing, retainers, covers, and miscellaneous seals previously described. A plurality of fluid chambers similar to that illustrated in FIG. 1 may be combined, as suggested in the Triplex fluid end housing schematically illustrated in FIG. 2. It is common practice for the centerline of the plunger bore and access bore to be collinear. Typically in the prior art, the centerlines of the plunger bore, discharge bore, suction bore, and access bore are all arranged in a common plane. The spacing of the plunger bores, plungers, plunger packing, and plunger gland nut within each fluid chamber is fixed by the spacing of the crank throws and crank bearings on the crankshaft in the power end of the pump.
Engineers typically design high-pressure oil field plunger pumps with internal discharge manifolds as shown in FIGS. 1 and 2. As shown in FIG. 2, the internal discharge manifold penetrates both ends of the fluid end block, provisions are made for a pipe or line connection on both ends of the block. Typically with small plunger sizes, one end is fitted with a blind flange to seal off the end fitted with the blind flange, thus all fluid flow is directed through the opposite end of the manifold. For large plungers or fluid ends with more than three plungers, a connection is added at both ends of the manifold, thus fluid flow is in both directions. For pumps fitted with discharge lines from both ends of the manifold, the discharge fluid flow is then collected from both ends of the manifold along with discharge flow from additional pumps into a larger manifold downstream from the pump. This downstream manifold then combines all the incoming fluid flow into one outlet to direct the combined flow into the oil well.
Valve terminology varies according to the industry (e.g., pipeline or oil field service) in which the valve is used. In some applications, the term “valve” means just the valve body, which reversibly seals against the valve seat. In other applications, the term “valve” includes components in addition to the valve body, such as the valve seat and the housing that contains the valve body and valve seat. A valve as described herein comprises a valve body and a corresponding valve seat, the valve body typically incorporating an elastomeric seal within a peripheral seal retention groove.
Valves can be mounted in the fluid end of a high-pressure pump incorporating positive displacement pistons or plungers in multiple cylinders. Such valves typically experience high pressures and repetitive impact loading of the valve body and valve seat. These severe operating conditions have in the past often resulted in leakage and/or premature valve failure due to metal wear and fatigue. In overcoming such failure modes, special attention is focused on valve sealing surfaces (contact areas) where the valve body contacts the valve seat intermittently for reversibly blocking fluid flow through a valve.
Valve sealing surfaces are subject to exceptionally harsh conditions in exploring and drilling for oil and gas, as well as in their production. For example, producers often must resort to “enhanced recovery” methods to insure that an oil well is producing at a rate that is profitable. And one of the most common methods of enhancing recovery from an oil well is known as fracturing. During fracturing, cracks are created in the rock of an oil bearing formation by application of high hydraulic pressure. Immediately following fracturing, a slurry comprising sand and/or other particulate material is pumped into the cracks under high pressure so they will remain propped open after hydraulic pressure is released from the well. With the cracks thus held open, the flow of oil through the rock formation toward the well is usually increased.
The industry term for particulate material in the slurry used to prop open the cracks created by fracturing is the propend. And in cases of very high pressures within a rock formation, the propend may comprise extremely small aluminum oxide spheres instead of sand. Aluminum oxide spheres may be preferred because their spherical shape gives them higher compressive strength than angular sand grains. Such high compressive strength is needed to withstand pressures tending to close cracks that were opened by fracturing. Unfortunately, both sand and aluminum oxide slurries are very abrasive, typically causing rapid wear of many component parts in the positive displacement plunger pumps through which they flow. Accelerated wear is particularly noticeable in plunger seals and in the suction (i.e., intake) and discharge valves of these pumps.
A valve (comprising a valve body and valve seat) that is representative of an example full open design valve and seat for a fracturing plunger pump is schematically illustrated in FIG. 3. The valve of FIG. 3 is shown in the open position. For each valve, back pressure tends to close the valve when downstream pressure exceeds upstream pressure. For example, when valve is used as a suction valve, back pressure is present on the valve during the pump plunger's pressure stroke (i.e., when internal pump pressure becomes higher than the pressure of the intake slurry stream. During each pressure stroke, when the intake slurry stream is thus blocked by a closed suction valve, internal pump pressure rises and slurry is discharged from the pump through a discharge valve. For a discharge valve, back pressure tending to close the valve arises whenever downstream pressure in the slurry stream (which remains relatively high) becomes greater than internal pump pressure (which is briefly reduced each time the pump plunger is withdrawn as more slurry is sucked into the pump through the open suction valve).
Typically the motion of the valve body is controlled by valve guide legs attached to the bottom or upstream side of the valve body as shown in FIG. 3. Unfortunately these guide legs are another source of accelerated valve and seat failure when pumping high sand slurry concentrations. FIG. 4A illustrates an old style valve design, circa 1970, in which the valve legs are forged into the upstream side of the valve body; typical of a Mission Service Master I design. FIG. 4B illustrates the slurry flow patterns around the leg; as can be seen in the figure, the downstream side of the legs generates considerable turbulence in the flow. The swirling turbulence in the sand slurry used in typical fracturing work results in sever abrasion of the metal valve body and the elastomeric insert seal, which quickly damages the seal, resulting in seal failure. Once the seal fails on the valve insert, the high pressure fluid on the downstream side of the valve escapes through the seal failure to the low pressure upstream side of the valve. Travelling from the very high pressure to the very low pressure side of the valve results in extreme velocities of the sand slurry, which rapidly erodes the metal valve body and the guide legs in the slurry's path; many times destroying the entire valve leg. Engineers typically recognize the beginning of this failure by four (4) erosion marks behind each leg on worn valves removed from the pump just prior to catastrophic failure in which one or more of the valve legs are completely destroyed by the high pressure erosion. The abraded seal and erosion of the metal valve body are also illustrated in FIG. 4B.
The development of the Roughneck valve design, circa 1983, and later the Mission Service Master II valve or the Novatech valve shown in FIG. 5A greatly improved the flow behind the legs. These designs featured streamlined legs which were achieved by inertia welding an investment guide leg casting to the valve body forging. The streamlined legs and the open area below the valve body and downstream of the guide legs eliminated much of the turbulence behind the guide legs. However in severe pumping environments with high pump rates and high slurry concentrations the problem described in the previous paragraph still existed as evidenced by the four (4) erosion marks and destroyed guide legs. FIG. 5B is a picture a valve of the prior art damaged by seal failure and severe erosion behind the guide legs.
The most obvious solution to the problem described above is the removal of the guide legs and somehow guide the motion of the valve by other means. Historically many attempts have been made utilizing top stem valves, illustrated in FIGS. 6A and 6B. FIG. 6A illustrates a cross section of a fluid end showing the fluid chamber, the suction fluid chamber and the discharge fluid chamber illustrated in FIG. 6B. However top stem design valves are inherently unstable in the open position, particularly the discharge valve in the discharge fluid chamber. Once pushed off center by hydraulic flow as illustrated in FIG. 7, the forces on the discharge valve tend to push the valve further off center. As the valve continues its cyclic repeating opening and closing, the sliding forces cause rapid and accelerating wear on the top stem guide.
FIG. 6B is a partial cross-section schematically illustrating fluid chamber of FIG. 6A in its closed position (i.e., with peripheral elastomeric seal held in symmetrical contact with valve seat by discharge valve spring). Note that top guide stem of discharge valve body is aligned in close sliding contact with top valve stem guide.
FIG. 8 schematically illustrates how misalignment of top guide stem is possible with excessive wear of top valve stem guide. Such excessive wear can occur because discharge valve body, including top guide stem, is typically made of steel that has been carburized to a hardness of about 60 Rockwell C. In contrast, the female guide for the top stem discharge valve, which is shown in FIG. 6B, is usually integral within discharge cover, is typically made of mild alloy steel with a hardness of about 30 Rockwell C. Thus the softer wall of the stem guide is worn away by sliding contact with the harder guide stem. This wear is accelerated by side loads on valve body that result when fluid flowing past the valve body changes its direction of flow into the discharge manifold. Eventually, top valve stem guide can be worn sufficiently to allow discharge valve leakage due to significant asymmetric contact of elastomeric seal with valve seat as schematically illustrated in FIG. 8.
The change of direction of the fluid and the generated side loads is most severe for the discharge valve where the fluid must make a 90 degree change of direction into the discharge manifold immediately after the fluid exits the seat as shown by the heavy dashed lines in FIG. 7. Because the fluid must take the most direct path and the path with least obstructions, most of the fluid flows through one side of the valve as shown in FIG. 8.
The problem of stem guide wear is typically addressed in practice through use of a replaceable bushing having a modified top valve stem guide (see the schematic illustration in FIG. 9). Bushing is commonly made of a plastic such as urethane, or a wear and corrosion-resistant metal such as bronze. Such bushings require periodic checking and replacement, but these steps may be overlooked by pump mechanics until a valve fails prematurely.
When the open valve is badly misaligned and the valve guide is badly worn there are not aligning forces available to properly align the valve as it closes. Thus the valve will close against the seat in a miss-aligned or cocked position as shown in FIG. 8. In this position, the cocked valve leaves an extrusion gap that results in shorten valve insert seal life. The cocked valve also results in uneven loading of the metal valve body against the seating surface of the seat resulting in accelerated metal wear on the valve body and seat.